Wastewater treatment plants are designed in such a manner that the denitrification, depending on the expected wastewater composition, takes place in a first or later biological clarification stage which is here termed denitrification stage. If the wastewater itself has high nitrate loads and few other nitrogen compounds, the denitrification can customarily take place in the first stage of the clarification process. Frequently, the wastewater contains the nitrogen load in the form of nitrogen compounds such as ammonium, urea or organic nitrogen compounds. In this case, in a first biological clarification stage, the nitrification takes place, that is conversion of these nitrogen compounds to nitrate compounds. This takes place in air (aerobically) using corresponding bacteria in two stages via nitrite to nitrate. Denitrification proceeds in this case in a second biological clarification stage.
Customary denitrification stages are made up in such a manner that the liquid phase is removed from the denitrification stage, if appropriate after passage through a secondary clarification tank/settling tank, as clarified wastewater and a part of the sludge, that is the solid phase, is recirculated to the denitrification stage. The remaining sludge forms what is termed excess sludge. This excess sludge is either disposed of completely or subjected to further treatment and subsequently recirculated in whole or in part to the denitrification stage.
The bacteria in the denitrification stage require, for optimum denitrification performance, defined amounts of carbon, but also nitrogen and phosphorus. The required amounts of these elements varies with the wastewater composition, in particular with the nitrogen load. Frequently, the amount of C, N and P required for optimum denitrification is added directly from storage vessels. The required amounts of C, N and P are matched to the respective nitrogen load for optimum denitrification. However, it is also known to recover C, N and P from the organic fraction of the excess sludge.
EP 1 254 865 A1 discloses mechanically disintegrating excess sludge and subsequently treating it with ozone and recirculating the liquid phase of the treated excess sludge to the denitrification stage. In the ozone treatment stage, ozone is added in an amount of 100 to 150 g/kg of dry matter.
U.S. Pat. No. 6,337,020 B1 discloses the treatment of excess sludge from a biological treatment stage by mechanical disintegration and addition of ozone. The treated excess sludge is completely recirculated to the biological treatment stage. In the ozone treatment stage, ozone is added in an amount of 1 to 20 g/kg of dry matter and the pH is 6 to 9.